How to get all available entries of an EnumProperty?
Enums are something Blender defines, which doesn't have a Python equivalent.
Therefor you need to use the RNA API. (See API docs)

Here is a function which returns possible values for an enum:

import bpy

def enum_members_from_type(rna_type, prop_str):
    prop = rna_type.bl_rna.properties[prop_str]
    return [e.identifier for e in prop.enum_items]


def enum_members_from_instance(rna_item, prop_str):
    return enum_members_from_type(type(rna_item), prop_str)


# Test the functions above:
enum = enum_members_from_instance(bpy.context.object, "rotation_mode")
print(enum)
# ['QUATERNION', 'XYZ', 'XZY', 'YXZ', 'YZX', 'ZXY', 'ZYX', 'AXIS_ANGLE']